Effect of additional phenothiazine donor and thiophene π-bridge on photovoltaic performance of quinoxaline cored photosensitizers

A series of quinoxaline-based sensitizers were designed through the elongation of π-conjugation length based on the sense of directional electron transport from donor to acceptor. To facilitate an efficient intramolecular charge transfer, we introduced additional donor moieties and thiophene unit as light-absorbing chromophore and π-bridge. The photophysical and electrochemical properties of the synthesized dyes were investigated, and photovoltaic performances of the solar cells based on the sensitizers were also explored. Under AM 1.5 G simulated solar illumination, the dye with a pair of dimeric phenothiazine donors gave the best overall power conversion efficiency of 5.41% owing to its intense and broad visible light absorption. The insertion of a thiophene bridge between the phenothiazine and the quinoxaline ring improved the photovoltaic efficiency by impeding charge recombination and increasing electron lifetime. Optimization of the photovoltaic performance condition gave an overall efficiency of 6.48% for NQX8. The results led us to conclude that proper π-conjugation length, location of π-spacer, and molecular conformation should be considered when designing novel sensitizer structures for dye-sensitized solar cells (DSSCs).